Master the concepts behind procedures, error handling, classes, and objects to create your foundation of Visual Basic .NET knowledge. From here, you will be able to get on your way to ascertaining full understanding of the language.

This chapter is from the book

Today, we're going to look at some crucial aspects of the Visual Basic
language: procedures such as Sub procedures and functions, procedure scope, and
exception (runtime error) handling. We'll also get an introduction to a
topic that's become central to Visual Basic: classes and objects.

Now that our code is growing larger, it's good to know about procedures,
which allow us to break up our code into manageable chunks. In fact, in Visual
Basic, all executable code must be in procedures. There are two types of
procedures: Sub procedures and functions. In Visual Basic, Sub
procedures do not return values when they terminate, but functions do.

If you declare variables in your new procedures, those variables might not be
accessible from outside the procedure, and that fact is new also. The area of
your program in which a data item is visible and can be accessed in code is
called scope, and we'll try to understand scopea crucial
aspect of object-oriented programmingin this chapter.

We'll also look at handling runtime errors today. In Visual Basic, a
runtime error is the same as an exception (that's not true in all
languages), so we're going to look at exception handling. We'll
see that there are two ways of heading off errors that happen at runtime before
they become problems.

Finally, we'll get an introduction to classes and objects in this
chapter. Visual Basic .NET programming is object-oriented programming (OOP), a
fact you need to understand in depth to be a Visual Basic programmer. Today,
we'll start by discussing classes and objects in preparation for our later
work (such as Day 9, "Object-Oriented Programming," which is all about
OOP). Here's an overview of today's topics:

Creating Sub procedures and functions

Passing arguments to procedures

Returning data from functions

Preserving data values between procedure calls

Understanding scope

Using unstructured exception handling

Using structured exception handling with Try/Catch

Using exception filtering in Catch blocks

Using multiple Catch statements

Throwing an exception

Throwing a custom exception

Understanding classes and objects

Supporting properties and methods in objects

All these topics are powerful ones, and they're all related. And today,
the best place to start is with Sub procedures.

Sub Procedures

Procedures give you a way to break up your Visual Basic code, which is
invaluable as that code grows longer and longer. Ideally, each procedure should
handle one discrete task. That way, you break up your code by task; having one
task per procedure makes it easier to keep in mind what each procedure does.

You can place a set of Visual Basic statements in a procedure, and when that
procedure is called, those statements will be run. You can pass
data to procedures for that code to work on and read that data in your code. The
two types of procedures in Visual Basic are Sub procedures and
functions, and both can read the data you pass them (the name Sub
procedure comes from the programming term subroutine). However, only
one type, functions, can also return data.

In fact, we've been creating Sub procedures in our code already (not
surprisingly, because all Visual Basic code has to be in a procedure). All the
code we developed yesterday went into the Sub procedure named Main,
created with the keyword Sub:

This Main Sub procedure is special because when a console
application starts, Visual Basic calls Main automatically to start the
program. When Main is called, the code is run as we wanted.

You can also create your own Sub procedures, giving them your own names.
Those names should give an indication of the procedure's task. For example,
to show the "Hi there!" message, you might create a new Sub
procedure named ShowMessage by simply typing this text into the code
designer:

And that's it! Now when you run this code, Visual Basic will call the
Main Sub procedure, which in turn will call the ShowMessage
Sub procedure, giving you the same result as before:

Hi there!
Press Enter to continue...

TIP

If you want to, you can use a Visual Basic Call statement to call a
Sub procedure like this: Call ShowMessage(). This usage is still
supported, although it goes back to the earliest days of Visual Basic, and
there's no real reason to use it here.

Note the parentheses at the end of the call to ShowMessage like
this: ShowMessage(). You use those parentheses to pass data to a
procedure, and we'll take a look at that task next.

Passing Data to Procedures

Say you want to pass the message text you want to display to the
ShowMessage Sub procedure, allowing you to display whatever message you
want. You can do that by passing a text string to ShowMessage, like
this:

A data item you pass to a procedure in parentheses this way is called an
argument. Now in ShowMessage, you must declare the type of the
argument passed to this procedure in the procedure's argument
list:

This creates a new string variable, Text, which you'll be able
to access in the procedure's code. The ByVal keyword here
indicates that the data is being passed by value, which is the default in
Visual Basic (you don't even have to type ByVal, just Text As
String here, and Visual Basic will add ByVal automatically).

Passing data by value means a copy of the data will be passed to the
procedure. The other way of passing data is by reference, where you use
the ByRef keyword. Passing by reference (which was the default in VB6)
meant that the location of the data in memory will be passed to the
procedure. Here's an important point to know: Because objects can become
very large in Visual Basic, making a copy of an object and passing that copy can
be very wasteful of memory, so objects are automatically passed by reference.
We'll discuss passing by value and passing by reference in more detail in a
page or two.

Visual Basic automatically fills the Text variable you declared in
the argument list in this example with the string data passed to the procedure.
This means you can access that data as you would the data in any other variable,
as you see in the SubProcedures project in the code for this book, as shown in
Listing 3.1.

And that's all you need! Now you're passing data to Sub procedures
and retrieving that data in the procedure's code. You can pass more than
one argument to procedures as long as you declare each argument in the
procedure's argument list. For example, say you want to pass the string to
show and the number of times to show it to ShowMessage; that code might
look like this:

Module Module1
Sub Main()
ShowMessage("Hi there!", 3)
Console.WriteLine("Press Enter to continue...")
Console.ReadLine()
End Sub
Sub ShowMessage(ByVal Text As String, ByVal Times As Integer)
For intLoopIndex As Integer = 1 To Times
Console.WriteLine(Text)
Next intLoopIndex
End Sub
End Module

Here's the result of this code:

Hi there!
Hi there!
Hi there!
Press Enter to continue...

If you pass arguments by reference, using the ByRef keyword, Visual
Basic passes the memory location of the passed data to the procedure (which
gives the code in that procedure access to that data). You can read that data
just as you do when you pass arguments by value:

Sub ShowMessage(ByRef Text As String, ByRef Times As Integer)
For intLoopIndex As Integer = 1 To Times
Console.WriteLine(Text)
Next intLoopIndex
End Sub

The code in the procedure has access to the data's location in memory,
however, and that's something to keep in mind. So far, we've passed
two literals ("Hello there!" and 3) to
ShowMessage, and literals don't correspond to memory locations.
But see what happens if you pass a variable by reference, like this:

After this code is finished executing, for example, the variable
NumberOfTimes will be left holding 24. This side effect is not
unintentional; it's intentional. Being able to change the value of
arguments is a primary reason to pass arguments by reference.

Changing the value of arguments passed by reference is one way to pass data
from a procedure back to the calling code, but it can be troublesome. You can
easily change an argument's value unintentionally, for example. A more
structured way of passing data back from procedures is to use functions, which
is the next topic.

You should also know that an Exit Sub statement, if you use one,
causes an immediate exit from a Sub procedure in case you want to leave before
executing all code. For example, say you have a Sub procedure that displays
reciprocals of numbers you pass to it, but you want to avoid trying to find the
reciprocal of 0. You could display an error message and exit the procedure like
this if 0 is passed to the procedure:

And like other Visual Basic statements, many of the keywords here won't
make sense at this point, so you can treat this information as reference
material to come back to later. (Many of the keywords here deal with OOP, but we
can't cover OOP in the detail needed here before knowing how to work with
procedures, so it's impossible to avoid slightly circular definitions.) The
parts of this statement are as follows:

attrlistThis is an advanced (and optional) topic;
this is a list of attributes for use with this procedure. Attributes can add
more information about the procedure, such as copyright data and so on. You
separate multiple attributes with commas.

OverloadsSpecifies that this Sub procedure overloads one
(or more) procedures defined with the same name in a base class. An overloaded
procedure has multiple versions, each with a different argument list, as
we'll see in Day 9. The argument list must be different from the argument
list of every procedure that is to be overloaded. You cannot specify both
Overloads and Shadows in the same procedure
declaration.

OverridesSpecifies that this Sub procedure overrides
(replaces) a procedure with the same name in a base class. The number and data
types of the arguments must match those of the procedure in the base
class.

OverridableSpecifies that this Sub procedure can be
overridden by a procedure with the same name in a derived class.

NotOverridableSpecifies that this Sub procedure may not be
overridden in a derived class.

MustOverrideSpecifies that this Sub procedure is not
implemented. This procedure must be implemented in a derived class.

ShadowsMakes this Sub procedure a shadow of an identically
named programming element in a base class. You can use Shadows only at
module, namespace, or file level (but not inside a procedure). You cannot
specify both Overloads and Shadows in the same procedure
declaration.

SharedSpecifies that this Sub procedure is a shared
procedure. As a shared procedure, it is not associated with a specific object,
and you can call it using the class or structure name.

PublicProcedures declared Public have public
access. There are no restrictions on the accessibility of public
procedures.

ProtectedProcedures declared Protected have
protected access. They are accessible only from within their own class or from a
derived class. You can specify Protected access only for members of
classes.

FriendProcedures declared Friend have friend
access. They are accessible from within the program that contains their
declaration and from anywhere else in the same assembly.

Protected FriendProcedures declared Protected
Friend have both protected and friend accessibility. They can be used by
code in the same assembly, as well as by code in derived classes.

PrivateProcedures declared Private have private
access. They are accessible only within the element in which they're
declared.

nameSpecifies the name of the Sub
procedure.

arglistLists expressions representing arguments
that are passed to the Sub procedure when it is called. You separate multiple
arguments with commas.

Implements interface.definednameIndicates that this
Sub procedure implements an interface. We'll see interfaces, which allow
you to derive one class from several others, in Day 9.

statementsSpecifies the block of statements to be
executed within the Sub procedure.

In addition, each argument in the argument list, arglist, has
this syntax:

attrlistLists (optional) attributes that apply to
this argument. Multiple attributes are separated by commas.

OptionalSpecifies that this argument is not required when
the procedure is called. If you use this keyword, all following arguments in
arglist must also be optional and be declared using the
Optional keyword. Every optional argument declaration must supply a
defaultvalue. Optional cannot be used for any argument
if you also use ParamArray.

ByValSpecifies passing by value. ByVal is the
default in Visual Basic.

ByRefSpecifies passing by reference, which means the
procedure code can modify the value of the original variable in the calling
code.

ParamArrayActs as the last argument in
arglist to indicate that the final argument is an optional array
of elements of the specified type. The ParamArray keyword allows you to
pass an arbitrary number of arguments to the procedure. ParamArray
arguments are always passed by value.

argnameSpecifies the name of the variable
representing the argument.

argtypeSpecifies the data type of the argument
passed to the procedure; this part is optional unless Option Strict is
set to On. It can be Boolean, Byte, Char,
Date, Decimal, Double, Integer,
Long, Object, Short, Single, or
String, or the name of an enumeration, structure, class, or
interface.

defaultvalueSpecifies the default value for an
optional argument, required for all optional arguments. It can be any constant
or constant expression that evaluates to the data type of the argument. Note
that if the type is Object, or a class, interface, array, or structure,
the default value must be Nothing.

That gives us what we need to know about Sub procedures, we'll move on
to functions next.